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Accuracy of SQM website

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#1 Araguaia

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Posted 23 February 2019 - 04:18 AM

According to this website: https://www.lightpol...ers=B0FFFFTFFFF

 

... I live at an SQM 21.99 site.  A few km to the south, it goes to SQM 22.

 

I have no meter.  Those of you who do, do the readings generally agree with websites like that?

 

 



#2 BFaucett

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Posted 23 February 2019 - 05:10 AM

I just checked my location and I got variances of a few hundredths just by clicking on various locations near my house (around 200 meters or so from my house according to their displayed scale).  Example:  17.57 to 17.71

 

I'm interested in the feedback from others that have more experience with this.

 

Cheers!  Bob F.



#3 ngc7319_20

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Posted 23 February 2019 - 07:29 AM

I just checked my location and I got variances of a few hundredths just by clicking on various locations near my house (around 200 meters or so from my house according to their displayed scale).  Example:  17.57 to 17.71

 

I'm interested in the feedback from others that have more experience with this.

 

Cheers!  Bob F.

You are very lucky with SQM 22.0 !!!

 

A couple notes:

 

I have only found a general agreement between the meters and maps.  Darker areas on the map are generally darker in real life, but the correspondence to detailed meter readings is sometimes poor and differ by several tenths of a magnitude.

 

There are several types of map available.  You should understand which type you are looking at.  Some show the earth brightness as observed from satellites.  Others have that, but also a model of atmospheric scattering and altitude applied, which attempts to more accurately replicate what an observer on the ground would see.

 

I have 3 meters and it is common for them to give different readings by 0 to 0.4 magnitude on the same scene when readings are taken in rapid sequence.  The differences seem to depend on the color of the light source (say a dim indoor lamp, vs. scattered moonlight,  vs say light pollution).   It may be an issue with the color filters inside the meters, but I am not sure.

 

That being said, each meter is pretty consistent to like 0.01 or 0.02 magnitude on different consecutive readings (within a minute or so) of the same scene.  In that sense the meters are pretty accurate.

 

If you take a number of readings all in a row on the same light source, the first reading is usually different from the rest and should be discarded. Unihedron had some electronic explanation for this, but I forget what it was -- maybe something in the meter heats up.  So you should always take at least 2 readings in a row, and discard the first reading.  I typically will take 6 readings -- discarding the first and averaging two through six.  This also gives me some idea of the random noise in the readings.


Edited by ngc7319_20, 23 February 2019 - 07:35 AM.

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#4 Tony Flanders

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Posted 23 February 2019 - 07:46 AM

I certainly wouldn't trust any map farther than I can throw it. Maps are a good guide if you're searching for darker locations, but every site has to be assessed on its own merits. Personally, I'm very sceptical of SQM measurements in the 22.0 range. Such skies have been reported by reliable observers, but only in superb locations like Chile or Namibia. It is, sadly, very easy to get a "bad" SQM reading, usually by failing to realize that trees are blocking a good chunk of the sky.

 

Even if your view of the sky is completely unobstructed, pointing an SQM in a slightly different direction in the sky can alter a reading by 0.05 magnitude or more in some cases.

 

Having said all that, if anybody ever reported SQM = 22.0 in your part of the world, you surely have very good skies. Unless you point an SQM at the ground, you simply can't get such a good reading unless the sky is pretty dark.


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#5 Araguaia

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Posted 23 February 2019 - 08:46 AM

I have 3 meters and it is common for them to give different readings by 0 to 0.4 magnitude on the same scene when readings are taken in rapid sequence. 

 

 

So the meters are accurate for comparisons as long as you use the same meter... two different people with two different meters at two different sites means about 0.5 magnitude error brackets, then?



#6 ngc7319_20

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Posted 23 February 2019 - 09:06 AM

So the meters are accurate for comparisons as long as you use the same meter... two different people with two different meters at two different sites means about 0.5 magnitude error brackets, then?

Yes, that is the conclusion I would draw from my own personal tests using 3 meters.  It seems the consistency from one meter to another is not very good. 

 

I suppose it is possible that I have some bad meters.  Or maybe they change with age -- mine are more than 5 years old.  I wish I had a good explanation.  I bought a second meter to test the first, and it gave different readings.  I bought a 3rd to test the first two, and those readings were different from the other two.  It is a bit discouraging.  One potential cause I can see, is that the little color filter mounted inside in front of the detector is too small to cover the entire detector, and sometimes the filter is shifted one way or another, possibly letting in the "wrong color" light.

 

I wonder if at some star party the SQM owners have gotten together and compared their meters for readings taken at exactly the same time.  It would be an interesting exercise.



#7 Kent10

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Posted 23 February 2019 - 10:03 AM

On the map my house reads 21.36.  The highest reading from my SQM has been 21.25-21.27.  Generally I will get less than this.  The 21.25 happened once.  My readings generally range from 20.8 - 21.1 or so.



#8 Araguaia

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Posted 23 February 2019 - 10:31 AM

On the map my house reads 21.36.  The highest reading from my SQM has been 21.25-21.27.  Generally I will get less than this.  The 21.25 happened once.  My readings generally range from 20.8 - 21.1 or so.

Can you notice a useful difference between 21 and 21.25?



#9 Allan Wade

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Posted 23 February 2019 - 10:36 AM

The one issue with websites like that, is they provide a single reading. But the sky darkness changes significantly throughout the year, especially in the southern hemisphere under the influence of the Milky Way.

 

Having said that, the website is remarkably close to the darkest readings I get for both my astro property and my city home. I get SQM's around 22.0 on my wide meter at the end of the year when the Milky Way is laying flat on the horizon, and that is the figure given for my astro place. However during the Milky Way's greatest influence on the sky brightness, my SQM readings max out at 21.70. Visually looking at the sky and environment there is a huge difference between 22.0 and 21.7.


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#10 Araguaia

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Posted 23 February 2019 - 10:38 AM


 

 Personally, I'm very sceptical of SQM measurements in the 22.0 range. 

 

Makes sense.  If just south of here is 22, then what is the remotest Atacama? 

 

We are at only 193 meters above sea level, and the trade wind brings dry air but over land.  Plus there is that small light dome in the distance.  Brazilian small towns are not nearly as lit up as American small towns of even 40 years ago, but it is only 37 km away in a straight line (I wish them decades of no growth and tight municipal budgets..).   I don't buy the 21.99 map figure for here either.

 

 



#11 Kent10

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Posted 23 February 2019 - 10:39 AM

Can you notice a useful difference between 21 and 21.25?

Yes, I think the difference of .25 is quite significant.  In my notes, I have listed the nights of 21.1 and higher as great nights.  Then I go to the Grand Canyon area just 1.5 hours from me and get readings of 21.5-21.74 and the views are completely different for me.  Much nicer.  According to the map, 15 minutes from my house are readings of 21.9.  It probably isn't that dark all the time but I can imagine it is quite dark unless looking towards the city.

 

Final readings are difficult to make because there is variation in the different parts of the sky.  I live at the edge of a small city.  If I point the SQM near the lights of the city I get much lower readings.  I try to take my readings in the same area at Zenith but if I move the SQM to different areas of the sky I am going to get different readings.  And also depending where the Milky Way is located which will give lower readings.  I take multiple readings near Zenith and average all of them.


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#12 Araguaia

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Posted 23 February 2019 - 10:43 AM

 I get SQM's around 22.0 on my wide meter at the end of the year when the Milky Way is laying flat on the horizon, and that is the figure given for my astro place. However during the Milky Way's greatest influence on the sky brightness, my SQM readings max out at 21.70. Visually looking at the sky and environment there is a huge difference between 22.0 and 21.7.

OK, that answers my question.  I can see the difference between when Sagittarius is at meridian and after it sets on a good night.  That, then, is SQM 0.3.

 

My hateful little light dome is about as bright as the Sagittarius Milky Way at about 10 degrees above the horizon.  So the sky in that direction should be around SQM 21.7.  How about if you take a reading at 90o from the center of the Milky Way?  How much does the reading improve?



#13 Allan Wade

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Posted 23 February 2019 - 11:04 AM

OK, that answers my question.  I can see the difference between when Sagittarius is at meridian and after it sets on a good night.  That, then, is SQM 0.3.

 

My hateful little light dome is about as bright as the Sagittarius Milky Way at about 10 degrees above the horizon.  So the sky in that direction should be around SQM 21.7.  How about if you take a reading at 90o from the center of the Milky Way?  How much does the reading improve?

Considering my experience the sky is at least 0.3 MPSAS darker at 90 degrees from the centre of the Milky Way. That is using my standard SQM metre which samples a large area of the sky. In my experience that metre hits a wall at 22.0 in the very darkest of skies. I have seen reports of readings slightly above 22.0, but they have always been with the narrow, lens version SQM metre. 



#14 Starman1

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Posted 23 February 2019 - 02:10 PM

Yes, that is the conclusion I would draw from my own personal tests using 3 meters.  It seems the consistency from one meter to another is not very good. 

 

I suppose it is possible that I have some bad meters.  Or maybe they change with age -- mine are more than 5 years old.  I wish I had a good explanation.  I bought a second meter to test the first, and it gave different readings.  I bought a 3rd to test the first two, and those readings were different from the other two.  It is a bit discouraging.  One potential cause I can see, is that the little color filter mounted inside in front of the detector is too small to cover the entire detector, and sometimes the filter is shifted one way or another, possibly letting in the "wrong color" light.

 

I wonder if at some star party the SQM owners have gotten together and compared their meters for readings taken at exactly the same time.  It would be an interesting exercise.

I observe at a site where sometimes up to 5 or 6 people will take SQM readings.

We usually cluster within about 0.1 magnitude of each other.

Tekatch of Unihedron says there is nothing to age or get out of alignment in the SQM, so the readings will be consistent over years.

A weak battery will start to give irregular readings, though.

 

The SQM-L will give entirely different readings.  If the Milky Way is at the zenith, it will yield a brighter reading than the SQM.

If the Milky Way is near the horizon, the SQM-L will give a substantially darker reading than the SQM.

So what cannot be compared is SQM readings with SQM-L readings.

Getting a 22.0 reading on the SQM-L is not hard--I got that reading on an SQM-L at Kitt Peak, but the SQM read 21.42 because of the light from Tucson.

If you get a reading of 22.0 on an SQM, you are likely at a pristine site.  The darkest I've seen was a 21.95 at a high altitude site where ALL light down below was covered up to 6000' by clouds for a hundred miles in every direction.

Ironically, that was only 90 miles from downtown Los Angeles.  That site normally yields a 21.4 at the zenith with the SQM and 21.7 with the SQM-L.

(I didn't get an SQM-L reading on that spectacular night).


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#15 Araguaia

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Posted 23 February 2019 - 02:26 PM

What matters for observing is SQM-L, right?  If the brighter part of the sky is to the east or west, you can observe every object as it crosses the dark part.



#16 Starman1

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Posted 23 February 2019 - 02:36 PM

For evaluating sites, though, I prefer the SQM, as it gives a better overall measurement of the sky.

I try to avoid sites where only half the sky is dark.

There is one site that measures 21.6 at the zenith with the SQM-L but which has a light dome on the SW horizon that is so bright it casts strong shadows.

You can't even dark adapt well because the sky is so bright in that direction.

The SQM reads about 21.1 for the site, which is more indicative of the site's overall sky brightness.


Edited by Starman1, 24 February 2019 - 10:24 AM.

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#17 Eddgie

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Posted 23 February 2019 - 03:27 PM

 

The SQM-L will give entirely different readings.  If the Milky Way is at the zenith, it will yield a brighter reading than the SQM.

If the Milky Way is near the horizon, the SQM-L will give a substantially darker reading than the SQM.

 

I actually prefer the SQM-L for this reason..  I live 4.5 miles north of down town Austin Tex, and if I take SQM reading it can vary greatly depending on the direction.  Now the same is true of the SQM-L in that I get much higher reading if I point it south, but with SQM-L, if I want to report an observation, I can aim at the specific portion of the sky where the target is located.  This way, when I report my observation, people will know what the sky quality was in the sky around the object, regardless of where that object is in the sky.

 

I think for dark skies though, SQM is probably better.  For LP skies or places with light domes neaby, SQM-L might be preferred for people that post observations.   


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#18 Starman1

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Posted 23 February 2019 - 03:36 PM

As long as they specify which SQM took the reading.



#19 Eddgie

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Posted 23 February 2019 - 03:44 PM

To the OP, I think the real value of the more permanent SQM installations is that they readings are done with high degree of repeatability.   The meters are mounted, so the orientation is always the same, and usually the installations are protected from man made local lights.

    

What is not important to me is that my meter agrees exactly with the site meter, but rather that every report from the site is measured against itself with a high degree of repeatability. If I know a site typically has a 21.6 report and I see 20.5, I know something is wrong.  Since the same meter is always used, and the same orientation of the meter is always constant, and the external environment is managed to avoid local light source error, if the reading deviates from what is typical for the site, you know something is up. 

 

I do report SQM-L for some observations when I am at home, but that is just so other NV observers will know how my sky was in the small area of the target.  I use SQM-l for exactly that reason.   I have ball field lights to the east of me, and these can greatly alter the SQM in that part of the sky depending on whether games are playing.  to the south, I have large university stadium, and same thing.   SQM would be useless, so I use SQM-L.   People in the forum I go to often view under similar difficult light environments so at least they have a more specific report on the area where the target was.  


Edited by Eddgie, 23 February 2019 - 03:44 PM.


#20 Redbetter

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Posted 24 February 2019 - 03:06 AM

Getting a 22.0 reading on the SQM-L is not hard

Not from what I have seen with my SQM-L.  I have never had a dark sky reading that high with the SQM-L.   I have had a few valid consistent measurements in the 21.85+ range (not counting the initial reading), but those have been from places like Grandview Campground in the White Mountains before the Southern Milky Way was up high, and at nearly the same altitude 45 miles to the west of the campground (as the crow flies) in the center of the Sierra range. 

 

I would expect Kitt Peak to be considerably brighter than either of those, and probably even brighter than my nearer Bortle 2 site at about 6,400 feet--and that one, although dark, is definitely a step below the other two.  In fact I can get about 0.1 MPSAS darker going up another 800 feet and a few miles further back over the ridge using a 4WD road (when snow free), and that is still not as dark as either of the other two, which are nearer to pristine. 

 

* I have had a 21.99 reading...when I was within a thin cloud at the 6,400 feet site.  Of course, I can get higher readings pointing at the ground, or trees, or structures or within the cab of the truck.  As for SQM, I wouldn't trust SQM readings from several of the sites I use because of the presence of tall trees, and because most of what I observe is higher in the sky.  SQM won't tell me what the background brightness is where the scope is pointing, the SQM-L will, and that information and zenith are more important to me than a band extending much closer to the horizon.  I already know areas in one quadrant are brighter because of city glow. 



#21 Redbetter

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Posted 24 February 2019 - 04:11 AM

So the meters are accurate for comparisons as long as you use the same meter... two different people with two different meters at two different sites means about 0.5 magnitude error brackets, then?

That sort of error seems unrealistically large, at least if one is comparing the same type of meter at the same place and time pointed at the same part of the sky.  If they were frequently off by even 0.2 MPSAS then at least one of them needs calibration.  An SQM to SQM-L comparison is a different matter, apples to oranges.  

 

An SQM won't tell me how dark the sky is at zenith or in any given direction.  It has the same basic flaw as the Bortle scale, it is too heavily influenced by the horizon/low elevation...a direction I am not pointing the scope to observe DSO's.  I can deal with some sky glow in a quadrant if 3/4 of the sky is very dark and the transparency is good.  Of course, if the transparency is poor, then the overhead reading will be compromised as well. 

 

It is possible that my meter is an outlier that reads low, but if so it is unlikely to be off by more than 0.15 MPSAS.  If I had an SQM-L reading 22.0+ at zenith from any of the sites I mentioned earlier I would be concerned it was reading high.  I know that it will read somewhere over 23.5 MPSAS in an extremely dark room pointed about a foot from a fluorescent fixture that has been off for several limits, but still has a glow to the eye.  That is about as far as I have managed to take it before the meter would time out with an off scale indication. 



#22 krneki

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Posted 24 February 2019 - 08:01 AM

Just to clarify a few things regarding the ATLAS 2015 layer on www.lightpollutionmap.info
This layer predicts the ZENITH magnitude when the sun is at solar minimum at 1am local time (link to paper: http://advances.scie...nt/2/6/e1600377). In other words the best case scenario. In my experience the magnitude of the sky at the same location fluctuates easily by 0.5 magnitude (having a cloudless and a moonless night).


Edited by krneki, 24 February 2019 - 08:02 AM.

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#23 Starman1

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Posted 24 February 2019 - 10:41 AM

Just to clarify a few things regarding the ATLAS 2015 layer on www.lightpollutionmap.info
This layer predicts the ZENITH magnitude when the sun is at solar minimum at 1am local time (link to paper: http://advances.scie...nt/2/6/e1600377). In other words the best case scenario. In my experience the magnitude of the sky at the same location fluctuates easily by 0.5 magnitude (having a cloudless and a moonless night).

I like this map.

The 2015 Atlas shows the general glow in the sky, while the VIRS 2018 shows the local sources of light.



#24 Special Ed

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Posted 24 February 2019 - 12:53 PM

Just to clarify a few things regarding the ATLAS 2015 layer on www.lightpollutionmap.info
This layer predicts the ZENITH magnitude when the sun is at solar minimum at 1am local time (link to paper: http://advances.scie...nt/2/6/e1600377). In other words the best case scenario. In my experience the magnitude of the sky at the same location fluctuates easily by 0.5 magnitude (having a cloudless and a moonless night).

My experience pretty much confirms what krneki says.  The light pollution map shows my location (deep in the Appalachian Mountains) at 21.90 mpsas.  I have taken readings with my SQM-L for the last three and a half years at my observatory and they range from 21.28-21.68 mpsas depending on the season, transparency, position of the MW, airglow, etc.  I've never had a measurement of 21.90.

 

I had an SQM briefly before I got the SQM-L and discovered that it gave me false readings (e.g., 23.00) because of the ridges and mountain on three sides of my observatory, so be aware of obstructions if you use an SQM.

 

It's also important that your meter be at ambient temperature in order to give accurate readings.  It will tell you what it thinks the temperature is if you hold the button down.  I then compare that to the thermometer I keep at the observatory.


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#25 Redbetter

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Posted 24 February 2019 - 02:02 PM

Per Unihedron's website the initial reading is off because it takes a few seconds for the energized circuit to come to temp.  It is a temperature compensated reading based on the circuit temperature.  I find that the 2nd reading rarely varies much from the 3rd, 4th, etc.

 

I keep my meter covered except when taking a reading.  Typically this means it is on my observing table, underneath the fold of a moving blanket I use as a dew shield and as wind protection for my atlases and log.  Air can flow underneath the table providing some check to sub-cooling.  The reason I do this is to avoid having the lens dew or frost, and to keep it from sub-cooling as much as it would setting out and radiating to the sky.   The temp seems to track reasonably well as the transition occurs from dew to frost conditions. 




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